Selective organ cooling apparatus and method

ABSTRACT

The present invention involves a selective organ heat transfer device having a flexible coaxial catheter capable of insertion into a selected feeding artery in the vascular system of a patient. A heat transfer element is attached to a distal portion of the catheter as well as a turbulence-enhancing element which is adapted to enhance turbulent blood flow along the heat transfer element. The heat transfer element may include the turbulence-enhancing element and/or a turbulence-enhancing element may be located proximal of the heat transfer element.

RELATED APPLICATIONS

This is a continuation-in-part of U.S. patent application Ser. No.09/215,038, filed on Dec. 16, 1998, and entitled “Inflatable Catheterfor Selective Organ Heating and Cooling and Method of Using the Same,”which is a continuation-in-part of: U.S. patent application Ser. No.09/103,342, filed on Jun. 23, 1998 U.S. Pat. No. 6,096,068, and entitled“Selective Organ Cooling Catheter and Method of Using the Same”; and acontinuation of U.S. patent application Ser. No. 09/047,012, filed onMar. 24, 1998, and entitled “Selective Organ Hypothermia Method andApparatus” U.S. Pat. No. 5,957,963; and co-pending U.S. patentapplication Ser. No. 09/052,545, filed on Mar. 31, 1998, and entitled“Circulating Fluid Hypothermia Method and Apparatus”. The disclosures ofeach of the above-identified applications are incorporated herein byreference.

BACKGROUND OF THE INVENTION

Field of the Invention—The present invention relates generally to themodification and control of the temperature of a selected body organ.More particularly, the invention relates to a method and intravascularapparatus for controlling organ temperature.

Background Information—Organs in the human body, such as the brain,kidney and heart, are maintained at a constant temperature ofapproximately 37° C. Hypothermia can be clinically defined as a corebody temperature of 35° C. or less. Hypothermia is sometimescharacterized further according to its severity. A body core temperaturein the range of 33° C. to 35° C. is described as mild hypothermia. Abody temperature of 28° C. to 32° C. is described as moderatehypothermia. A body core temperature in the range of 24° C. to 28° C. isdescribed as severe hypothermia.

Hypothermia is uniquely effective in reducing brain injury caused by avariety of neurological insults and may eventually play an importantrole in emergency brain resuscitation. Experimental evidence hasdemonstrated that cerebral cooling improves outcome after globalischemia, focal ischemia, or traumatic brain injury. For this reason,hypothermia may be induced in order to reduce the effect of certainbodily injuries to the brain as well as other organs.

Cerebral hypothermia has traditionally been accomplished through wholebody cooling to create a condition of total body hypothermia in therange of 20° C. to 30° C. However, the use of total body hypothermiarisks certain deleterious systematic vascular effects. For example,total body hypothermia may cause severe derangement of thecardiovascular system, including low cardiac output, elevated systematicresistance, and ventricular fibrillation. Other side effects includerenal failure, disseminated intravascular coagulation, and electrolytedisturbances. In addition to the undesirable side effects, total bodyhypothermia is difficult to administer.

Catheters have been developed which are inserted into the bloodstream ofthe patient in order to induce total body hypothermia. For example, U.S.Pat. No. 3,425,419 to Dato describes a method and apparatus of loweringand raising the temperature of the human body. The Dato patent isdirected to a method of inducing moderate hypothermia in a patient usinga metallic catheter. The metallic catheter has an inner passagewaythrough which a fluid, such as water, can be circulated. The catheter isinserted through the femoral vein and then through the inferior venacava as far as the right atrium and the superior vena cava. The Datocatheter has an elongated cylindrical shape and is constructed fromstainless steel. By way of example, Dato suggests the use of a catheterapproximately 70 cm in length and approximately 6 mm in diameter.However, use of the Dato system implicates the negative effects of totalbody hypothermia described above.

Due to the problems associated with total body hypothermia, attemptshave been made to provide more selective cooling. For example, coolinghelmets or head gear have been used in an attempt to cool only the headrather than the patient's entire body. However, such methods rely onconductive heat transfer through the skull and into the brain. Onedrawback of using conductive heat transfer is that the process ofreducing the temperature of the brain is prolonged. Also, it isdifficult to precisely control the temperature of the brain when usingconduction due to the temperature gradient that must be establishedexternally in order to sufficiently lower the internal temperature. Inaddition, when using conduction to cool the brain, the face of thepatient is also subjected to severe hypothermia, increasing discomfortand the likelihood of negative side effects. It is known that profoundcooling of the face can cause similar cardiovascular side effects astotal body cooling. Further, from a practical standpoint, coolinghelmets and head gear are cumbersome and may make continued treatment ofthe patient difficult or impossible.

Selected organ hypothermia has been accomplished using extracorporealperfusion, as detailed by Arthur E. Schwartz, M.D. et al., in IsolatedCerebral Hypothermia by Single Carotid Artery Perfusion ofExtracorporeally Cooled Blood in Baboons, which appeared in Vol. 39, No.3, NEUROSURGERY 577 (September, 1996). In this study, blood wascontinually withdrawn from baboons through the femoral artery. The bloodwas cooled by a water bath and then infused through a common carotidartery with its external branches occluded. Using this method, normalheart rhythm, systemic arterial blood pressure and arterial blood gasvalues were maintained during the hypothermia. This study showed thatthe brain could be selectively cooled to temperatures of 20° C. withoutreducing the temperature of the entire body. However, externalcirculation of blood is not a practical approach for treating humansbecause the risk of infection, need for anticoagulation, and risk ofbleeding is too great. Further, this method requires cannulation of twovessels making it more cumbersome to perform, particularly in emergencysettings. Moreover, percutaneous cannulation of the carotid artery isdifficult and potentially fatal due to the associated arterial walltrauma. Finally, this method would be ineffective to cool other organs,such as the kidneys, because the feeding arteries cannot be directlycannulated percutaneously.

Selective organ hypothermia has also been attempted by perfusion of acold solution such as saline or perflourocarbons. This process iscommonly used to protect the heart during heart surgery and is referredto as cardioplegia. Perfusion of a cold solution has a number ofdrawbacks, including a limited time of administration due to excessivevolume accumulation, cost, and inconvenience of maintaining theperfusate and lack of effectiveness due to the temperature dilution fromthe blood. Temperature dilution by the blood is a particular problem inhigh blood flow organs such as the brain.

SUMMARY

The present invention involves an apparatus and method for controllingthe temperature of a selected organ such as the brain.

The present invention provides a system, which may be used toselectively control the temperature of a chosen organ, without inducingtotal body hypothermia. The apparatus, according to the invention, mayinclude a catheter having a heat transfer element attached to a distalportion thereof. The heat transfer element allows the fluid proximatethe selected organ to be cooled or heated. A turbulence-enhancingelement is also attached to a distal portion of the catheter and isadapted to enhance turbulent blood flow along the heat transfer element,so as to increase the efficiency of the heat transfer.

Other advantages, features, and objects of the invention are describedin the detailed description below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating the velocity of steady state turbulentflow as a function of time;

FIG. 2A is a graph showing the velocity of the blood flow within anartery as a function of time;

FIG. 2B is a graph illustrating the velocity of steady state turbulentflow under pulsatile conditions as a function of time, similar toarterial blood flow;

FIG. 2C is an elevation view of a turbulence inducing heat transferelement within an artery;

FIG. 3A is a velocity profile diagram showing a typical steady statePoiseuillean flow driven by a constant pressure gradient;

FIG. 3B is a velocity profile diagram showing blood flow velocity withinan artery, averaged over the duration of the cardiac pulse;

FIG. 3C is a velocity profile diagram showing blood flow velocity withinan artery, averaged over the duration of the cardiac pulse, afterinsertion of a smooth heat transfer element within the artery;

FIG. 4 is an elevation view of one embodiment of a heat transfer elementaccording to the invention;

FIG. 5 is longitudinal section view of the heat transfer element of FIG.4;

FIG. 6 is a transverse section view of the heat transfer element of FIG.4;

FIG. 7 is a perspective view of the heat transfer element of FIG. 4 inuse within a blood vessel;

FIG. 8 is a cut-away perspective view of an alternative embodiment of aheat transfer element according to the invention;

FIG. 9 is a transverse section view of the heat transfer element of FIG.8;

FIG. 10 is an elevation view of an embodiment of a heat transfermechanism according to the invention in use within a blood vessel;

FIG. 11 is a longitudinal cross-sectional view of the heat transfermechanism illustrated in FIG. 10;

FIG. 12 is an elevation view of an another embodiment of a heat transfermechanism according to the invention in use within a blood vessel;

FIG. 13 is an elevation view of an additional embodiment of a heattransfer mechanism according to the invention in use within a bloodvessel;

FIG. 14 is an elevation view of a further embodiment of a heat transfermechanism according to the invention in use within a blood vessel;

FIG. 15 is a perspective view of a heat transfer mechanism constructedin accordance with a still further embodiment of the invention in usewithin a blood vessel;

FIGS. 16A, 16B are transverse section views taken along lines 16A—16A,16B—16B of FIG. 15, with FIG. 16A illustrating the heat transfermechanism in a low-profile position and FIG. 16B illustrating the heattransfer mechanism in an expanded position;

FIG. 17 is a cross-sectional view, similar to FIGS. 16A and 16B, of anadditional embodiment of a heat transfer mechanism according to theinvention;

FIGS. 18A, 18B, 18C are elevation views of a further embodiment of aheat transfer mechanism according to the invention in use within a bloodvessel during various states of operation; and

FIG. 19 is a schematic representation of an embodiment of the inventionbeing used to cool the brain of a patient.

DETAILED DESCRIPTION OF THE INVENTION

In order to intravascularly regulate the temperature of a selectedorgan, a heat transfer element may be placed in the feeding artery ofthe organ to absorb or deliver the heat from or to the blood flowinginto the organ. The transfer of heat may cause either a cooling or aheating of the selected organ. The heat transfer element should be smallenough to fit within the feeding artery while still allowing sufficientblood flow to reach the organ in order to avoid ischemic organ damage.The heat transfer element should also provide the necessary heattransfer rate to produce the desired cooling or heating effect withinthe organ. By placing the heat transfer element within the feedingartery of an organ, the temperature of an organ can be controlledwithout significantly effecting the remaining parts of the body. Thesepoints can be illustrated by using brain cooling as an example.

The common carotid artery supplies blood to the head and brain. Theinternal carotid artery branches off of the common carotid to directlysupply blood to the brain. To selectively cool the brain, a heattransfer element may be placed into the common carotid artery, theinternal carotid artery, or both. The internal diameter of the commoncarotid artery ranges from 6 to 8 mm and the length ranges from 80 to120 mm. Thus, the heat transfer element residing in one of thesearteries should not be much larger than 4 mm in diameter in order toavoid occluding the vessel.

It is advantageous that the heat transfer element be flexible in orderto be placed within a small feeding artery of an organ. Feedingarteries, like the carotid artery, branch off the aorta at variouslevels. Subsidiary arteries continue to branch off the initial branches.For example, the internal carotid artery is a small diameter artery thatbranches off of the common carotid artery near the angle of the jaw.Because the heat transfer element is typically inserted into aperipheral artery, such as the femoral artery, and accesses the feedingartery by initially passing though a series of one or more of thesebranches, the flexibility of the heat transfer element is highlyadvantageous. Further, the heat transfer element is ideally constructedfrom a highly thermally conductive material, such as metal, in order tofacilitate heat transfer. The use of a highly thermally conductivematerial increases the heat transfer rate for a given temperaturedifferential between the heat transfer substance (e.g. coolant) withinthe heat transfer element and the blood. This facilitates the use of ahigher temperature coolant within the heat transfer element, allowingsafer coolants, such as water, to be used. Highly thermally conductivematerials, such as metals, tend to be rigid. Therefore, the design ofthe heat transfer element should facilitate flexibility in an inherentlyinflexible material.

In order to obtain the benefits of hypothermia described above, it isdesirable to reduce the temperature of the blood flowing to the brain tobetween 30° C. and 32° C. Given that a typical brain has a blood flowrate through each carotid artery (right and left) of approximately250-375 cubic centimeters per minute, it is desirable for the heattransfer element to absorb 75-175 Watts of heat when placed in one ofthe carotid arteries, to induce the desired cooling effect. It should benoted that smaller organs may have less blood flow in the supply arteryand may require less heat transfer, such as 25 Watts.

When a heat transfer element is inserted coaxially into an artery, theprimary mechanism of heat transfer between the surface of the heattransfer element and the blood is forced convection. Convection reliesupon the movement of fluid to transfer heat. Forced convection resultswhen an external force causes motion within the fluid. In the case ofarterial flow, the beating heart causes the motion of the blood aroundthe heat transfer element.

The magnitude of the heat transfer rate is proportional to the surfacearea of the heat transfer element, the temperature differential, and theheat transfer coefficient of the heat transfer element.

As noted above, the receiving artery into which the heat transferelement is placed has a limited diameter and length. Thus, surface areaof the heat transfer element must be limited to avoid significantobstruction of the artery, and to allow the heat transfer element toeasily pass through the vascular system. For placement within theinternal and common carotid artery, the cross sectional diameter of theheat transfer element is limited to about 4 mm, and its length islimited to approximately 10 cm.

The temperature differential can be increased, in the case of cooling,by decreasing the surface temperature of the heat transfer element.However, the minimum allowable surface temperature is limited by thecharacteristics of blood. Blood freezes at approximately 0° C. When theblood approaches freezing, ice emboli may form in the blood which maylodge downstream, causing serious ischemic injury. Furthermore, reducingthe temperature of the blood also increases its viscosity, which resultsin a small decrease in the value of the convection heat transfercoefficient. In addition, increased viscosity of the blood may result inan increase in the pressure drop within the artery, thus, compromisingthe flow of blood to the brain. Given the above constraints, it isadvantageous to limit the minimum allowable surface temperature of theheat transfer element to approximately 5° C. This results in a maximumtemperature differential between the blood stream and the heat transferelement of approximately 32° C., where the patient has a normal 37° C.temperature.

The mechanisms by which the value of the convection heat transfercoefficient may be increased are complex. However, it is well known thatthe convection heat transfer coefficient increases with the level ofturbulent kinetic energy in the fluid flow. Thus, it is advantageous tohave turbulent blood flow in contact with the heat transfer element.

FIG. 1 is a graph illustrating steady state turbulent flow. The verticalaxis is the velocity of the flow. The horizontal axis represents time.The average velocity of the turbulent flow is shown by a line 100. Theactual instantaneous velocity of the flow is shown by a curve 102.

Under constant pressure conditions, the flow in a pipe is Poiseuillean.FIG. 3A is a velocity profile diagram showing a typical steady statePoiseuillean flow driven by constant pressure. The velocity of the fluidacross the pipe is shown in FIG. 3A by the parabolic curve andcorresponding velocity vectors. The velocity of the fluid in contactwith the wall of the pipe is zero. The boundary layer is the region ofthe flow in contact with the pipe surface in which viscous stresses aredominant. In steady state Poiseuillean flow, the boundary layer developsuntil it reaches the pipe center line. For example, the boundary layerthickness in FIG. 3A is one half of the diameter of the pipe.

Under conditions of Poiseuillean flow, the Reynolds number (i.e. theratio of inertial forces to viscous forces) can be used to characterizethe level of turbulent kinetic energy. For Poiseuillean flows, Reynoldsnumbers must be greater than about 2300 to cause a laminar to turbulenttransition. Further, when the Reynolds number is greater than about2000, the boundary layer is receptive to “tripping”. Tripping is aprocess by which a small perturbation in the boundary layer can createturbulent conditions. The receptivity of a boundary layer to “tripping”is proportional to the Reynolds number and is nearly zero for Reynoldsnumbers less than 2000.

Blood flow in arteries is induced by the beating heart and is thereforepulsatile, complicating the fluid mechanics analysis. FIG. 2A is a graphshowing the velocity of the blood flow within an artery as a function oftime. The beating heart provides pulsatile flow with an approximateperiod of 0.5 to 1 second. This is known as the period of the cardiaccycle. The horizontal axis in FIG. 2A represents time in seconds and thevertical axis represents the average velocity of blood in centimetersper second. Although very high velocities are reached at the peak of thepulse, the high velocity occurs for only a small portion of the cycle.In fact, as shown in FIG. 2A, the velocity of the blood reaches zero inthe carotid artery at the end of a pulse and temporarily reverses.

Because of the relatively short duration of the cardiac pulse, the bloodflow in the arteries does not develop into classic Poiseuillean flow.FIG. 3B is a velocity profile diagram showing blood flow velocity withinan artery averaged over the cardiac pulse. The majority of the flowwithin the artery has the same velocity. The boundary layer where theflow velocity decays from the free stream value to zero is very thin,typically ⅙ to {fraction (1/20)} of the diameter of the artery, asopposed to one half of the diameter of the artery in the Poiseuilleanflow condition.

As noted above, if the flow in the artery were steady rather thanpulsatile, the transition from laminar to turbulent flow would occurwhen the value of the Reynolds number exceeds about 2000. However, inthe pulsatile arterial flow, the value of the Reynolds number variesduring the cardiac cycle, just as the flow velocity varies. In pulsatileflows, due to the enhanced stability associated with the acceleration ofthe free stream flow, the critical value of the Reynolds number at whichthe unstable modes of motion grow into turbulence is found to be muchhigher, perhaps as high as 9000.

The blood flow in the arteries of interest remains laminar over morethan 80% of the cardiac cycle. Referring again to FIG. 2A, the bloodflow is turbulent from approximately time t₁ until time t₂ during asmall portion of the descending systolic flow, which is less than 20% ofthe period of the cardiac cycle. If a heat transfer element is placedinside the artery, heat transfer will be facilitated during this shortinterval. However, to transfer the necessary heat to cool the brain,turbulent kinetic energy should be produced and sustained throughout theentire period of the cardiac cycle.

A thin boundary layer has been shown to form during the cardiac cycle.This boundary layer will form over the surface of a smooth heat transferelement. FIG. 3C is a velocity profile diagram showing blood flowvelocity within an artery, averaged over the cardiac pulse, afterinsertion of a smooth heat transfer element within the artery. In FIG.3C, the diameter of the heat transfer element is about one half of thediameter of the artery. Boundary layers develop adjacent to the heattransfer element as well as next to the walls of the artery. Each ofthese boundary layers has approximately the same thickness as theboundary layer which would have developed at the wall of the artery inthe absence of the heat transfer element. The free stream flow region isdeveloped in an annular ring around the heat transfer element.

One way to increase the heat transfer rate is to create a turbulentboundary layer on the heat transfer element surface. However, turbulencein the very thin boundary layer will not produce sufficient kineticenergy to produce the necessary heat transfer rate. Therefore, to inducesufficient turbulent kinetic energy to increase the heat transfer ratesufficiently to cool the brain, a stirring mechanism, which abruptlychanges the direction of velocity vectors, may be utilized. This cancreate high levels of turbulence intensity in the free stream, therebysufficiently increasing the heat transfer rate.

This turbulence intensity should ideally be sustained for a significantportion of the cardiac cycle. Further, turbulent kinetic energy shouldideally be created throughout the free stream and not just in theboundary layer. FIG. 2B is a graph illustrating the velocity ofcontinually turbulent flow under pulsatile conditions as a function oftime, which would result in optimal heat transfer in arterial bloodflow. Turbulent velocity fluctuations are seen throughout the cycle asopposed to the short interval of fluctuations seen in FIG. 2A betweentime t₁ and time t₂. These velocity fluctuations are found within thefree stream. The turbulence intensity shown in FIG. 2B is at least 0.05.In other words, the instantaneous velocity fluctuations deviate from themean velocity by at least 5%. Although, ideally, turbulence is createdthroughout the entire period of the cardiac cycle, the benefits ofturbulence are obtained if the turbulence is sustained for 75%, 50% oreven as low as 30% or 20% of the cardiac cycle.

To create the desired level of turbulence intensity in the blood freestream during the whole cardiac cycle, one embodiment of the inventionuses a modular design. This design creates helical blood flow andproduces a high level of turbulence in the free stream by periodicallyforcing abrupt changes in the direction of the helical blood flow. FIG.2C is a perspective view of such a turbulence inducing heat transferelement within an artery. Turbulent flow would be found at point 114, inthe free stream area The abrupt changes in flow direction are achievedthrough the use of a series of two or more heat transfer segments, eachcomprised of one or more helical ridges. To affect the free stream, thedepth of the helical ridge is larger than the thickness of the boundarylayer which would develop if the heat transfer element had a smoothcylindrical surface.

The use of periodic abrupt changes in the helical direction of the bloodflow in order to induce strong free stream turbulence may be illustratedwith reference to a common clothes washing machine. The rotor of awashing machine spins initially in one direction causing laminar flow.When the rotor abruptly reverses direction, significant turbulentkinetic energy is created within the entire wash basin as the changingcurrents cause random turbulent motion within the clothes-water slurry.

FIG. 4 is an elevation view of one embodiment of a heat transfer element14 according to the present invention. The heat transfer element 14 iscomprised of a series of elongated, articulated heat transfer segments20, 22, 24 and bellows 21 and 25. Three such segments are shown in thisembodiment, but two or more such segments could be used withoutdeparting from the spirit of the invention. As seen in FIG. 4, a firstelongated heat transfer segment 20 is located at the proximal end of theheat transfer element 14. A turbulence-inducing exterior surface of thesegment 20 comprises four parallel helical ridges 28 with four parallelhelical grooves 26 therebetween. One, two, three, or more parallelhelical ridges 28 could also be used and are within the scope of thepresent invention. In this embodiment, the helical ridges 28 and thehelical grooves 26 of the heat transfer segment 20 have a left handtwist, referred to herein as a counter-clockwise spiral or helicalrotation, as they proceed toward the distal end of the heat transfersegment 20.

The first heat transfer segment 20 is coupled to a second elongated heattransfer segment 22 by a first bellows section 21, which providesflexibility and compressibility. The second heat transfer segment 22comprises one or more helical ridges 32 with one or more helical grooves30 therebetween. The ridges 32 and grooves 30 have a right hand, orclockwise, twist as they proceed toward the distal end of the heattransfer segment 22. The second heat transfer segment 22 is coupled to athird elongated heat transfer segment 24 by a second bellows section 25.The third heat transfer segment 24 comprises one or more helical ridges36 with one or more helical grooves 34 therebetween. The helical ridge36 and the helical groove 34 have a left hand, or counter-clockwise,twist as they proceed toward the distal end of the heat transfer segment24. Thus, successive heat transfer segments 20,22,24 of the heattransfer element 14 alternate between having clockwise andcounterclockwise helical twists. The actual left or right hand twist ofany particular segment is immaterial, as long as adjacent segments haveopposite helical twist.

In addition, the rounded contours of the ridges 28,32,36 also allow theheat transfer element 14 to maintain a relatively atraumatic profile,thereby minimizing the possibility of damage to the blood vessel wall. Aheat transfer element according to the present invention may becomprised of two, three, or more heat transfer segments.

The bellows sections 21,25 are formed from seamless and nonporousmaterials, such as metal, and therefore are impermeable to gas, whichcan be particularly important, depending on the type of working fluidwhich is cycled through the heat transfer element 14. The structure ofthe bellows sections 21,25 allows them to bend, extend and compress,which increases the flexibility of the heat transfer element 14 so thatit is more readily able to navigate through blood vessels. The bellowssections 21,25 also provide for axial compression of the heat transferelement 14, which can limit the trauma when the distal end of the heattransfer element 14 abuts a blood vessel wall. The bellows sections21,25 are also able to tolerate cryogenic temperatures without a loss ofperformance.

The exterior surfaces of the heat transfer element 14 can be made frommetal, and may comprise very high thermally conductive material such asnickel, thereby, facilitating heat transfer. Alternatively, other metalssuch as stainless steel, titanium, aluminum, silver, copper and thelike, can be used, with or without an appropriate coating or treatmentto enhance biocompatibility or inhibit clot formation. Suitablebiocompatible coatings include, e.g., gold, platinum or polymerparalyene. The heat transfer element 14 may be manufactured by plating athin layer of metal on a mandrel that has the appropriate pattern. Inthis way, the heat transfer element 14 may be manufactured inexpensivelyin large quantities, which is an important feature for a disposablemedical device.

Because the heat transfer element 14 may dwell within the blood vesselfor extended periods of time, such as 24-48 hours or even longer, it maybe desirable to treat the surfaces of the heat transfer element 14 toavoid clot formation. In particular, one may wish to treat the bellowssections 21,25 because stagnation of the blood flow may occur in theconvolutions, thus, allowing clots to form and cling to the surface toform a thrombus. One means by which to prevent thrombus formation is tobind an antithrombogenic agent to the surface of the heat transferelement 14. For example, heparin is known to inhibit clot formation andis also known to be useful as a biocoating. Alternatively, the surfacesof the heat transfer element 14 may be bombarded with ions such asnitrogen. Bombardment with nitrogen can harden and smooth the surfaceand, thus, prevent adherence of clotting factors to the surface.

FIG. 5 is a longitudinal sectional view of the heat transfer element 14of the invention, taken along line 5—5 in FIG. 4. An inner tube 40creates an inner coaxial lumen 40 and an outer coaxial lumen 46 withinthe heat transfer element 14. Once the heat transfer element 14 is inplace in the blood vessel, a working fluid such as saline or otheraqueous solution may be circulated through the heat transfer element 14.Fluid flows from a working fluid chiller and pump, up a supply catheterand into the inner coaxial lumen 40. At the distal end of the heattransfer element 14, the working fluid exits the inner coaxial lumen 40and enters the outer lumen 46. As the working fluid flows through theouter lumen 46, heat is transferred from the working fluid to theexterior surface 37 of the heat transfer element 14. Because the heattransfer element 14 is constructed from highly conductive material, thetemperature of its exterior surface 37 may reach very close to thetemperature of the working fluid. The tube 42 may be formed as aninsulating divider, to thermally separate the inner lumen 40 from theouter lumen 46. For example, insulation may be achieved by creatinglongitudinal air channels in the wall of the insulating tube 42.Alternatively, the insulating tube 42 may be constructed of anon-thermally conductive material like polytetrafluoroethylene or someother polymer.

It is important to note that the same mechanisms that govern the heattransfer rate between the exterior surface 37 of the heat transferelement 14 and the blood also govern the heat transfer rate between theworking fluid and the interior surface 38 of the heat transfer element14. The heat transfer characteristics of the interior surface 38 isparticularly important when using water, saline or some other fluidwhich remains a liquid, as the coolant. Other coolants such as freonundergo nucleate boiling and create turbulence through a differentmechanism. Saline is a safe coolant because it is non-toxic, and leakageof saline does not result in a gas embolism, which could occur with theuse of boiling refrigerants. Since turbulence in the coolant is enhancedby the shape of the interior surface 38 of the heat transfer element 14,the coolant can be delivered to the heat transfer element 14 at a warmertemperature, and still achieve the necessary heat transfer rate.

This has a number of beneficial implications in the need for insulationalong the catheter shaft length. Due to the decreased need forinsulation, the catheter shaft diameter can be made smaller. Theenhanced heat transfer characteristics of the interior surface of theheat transfer element 14 also allow the working fluid to be delivered tothe heat transfer element 14 at lower flow rates and lower pressures.High pressures may make the heat transfer element stiff and cause it topush against the wall of the blood vessel, thereby shielding part of theexterior surface 37 of the heat transfer element 14 from the blood.Because of the increased heat transfer characteristics achieved by thealternating helical ridges 28,32,36, the pressure of the working fluidmay be as low as 5 atmospheres, 3 atmospheres, 2 atmospheres or evenless than 1 atmosphere.

FIG. 6 is a transverse sectional view of the heat transfer element 14 ofthe invention, taken along the line 6—6 in FIG. 4. In FIG. 6, thecoaxial construction of the heat transfer element 14 is clearly shown.The inner coaxial lumen 40 is defined by the insulating coaxial tube 42.The outer lumen 46 is defined by the exterior surface of the insulatingcoaxial tube 42 and the interior surface 38 of the heat transfer element14. In addition, the helical ridges 28 and helical grooves 26 may beseen in FIG. 6. As noted above, in the preferred embodiment, the depthof the grooves, d_(i), is greater than the boundary layer thicknesswhich would have developed if a cylindrical heat transfer element wereintroduced. For example, in a heat transfer element 14 with a 4 mm outerdiameter, the depth of the grooves, d_(i), may be approximately equal to1 mm if designed for use in the carotid artery. Although FIG. 6 showsfour ridges and four grooves, the number of ridges and grooves may vary.Thus, heat transfer elements with 1, 2, 3, 4, 5, 6, 7, 8 or more ridgesare specifically contemplated.

FIG. 7 is a perspective view of the heat transfer element 14 in usewithin a blood vessel. Beginning from the proximal end of the heattransfer element (not shown in FIG. 7), as the blood moves forwardduring the systolic pulse, the first helical heat transfer segment 20induces a counter-clockwise rotational inertia to the blood. As theblood reaches the second segment 22, the rotational direction of theinertia is reversed, causing turbulence within the blood. Further, asthe blood reaches the third segment 24, the rotational direction of theinertia is again reversed. The sudden changes in flow direction activelyreorient and randomize the velocity vectors, thus, ensuring turbulencethroughout the bloodstream. During turbulent flow, the velocity vectorsof the blood become more random and, in some cases, become perpendicularto the axis of the artery. In addition, as the velocity of the bloodwithin the artery decreases and reverses direction during the cardiaccycle, additional turbulence is induced and turbulent motion issustained throughout the duration of each pulse through the samemechanisms described above.

Thus, a large portion of the volume of warm blood in the vessel isactively brought in contact with the heat transfer element 14, where itcan be cooled by direct contact, rather than being cooled largely byconduction through adjacent laminar layers of blood. As noted above, thedepth of the grooves 26,30,34 is greater than the depth of the boundarylayer which would develop if a straight-walled heat transfer elementwere introduced into the blood stream. In this way, free streamturbulence is induced. In the preferred embodiment, in order to createthe desired level of turbulence in the entire blood stream during thewhole cardiac cycle, the heat transfer element 14 creates a turbulenceintensity greater than 0.05. The turbulence intensity may be greaterthan 0.055,0.06, 0.07 or up to 0.10 or 0.20 or greater. If the heattransfer element according to the invention were placed in a pipeapproximately the same size as an artery carrying a fluid having asimilar velocity, density and viscosity of blood and having a constant(rather than pulsatile) flow, Reynolds numbers of greater than 1,900,2,000, 2,100, 2,200 or even as much as 2,300, 2,400 or 2,600 or greaterwould be developed. Further, the design shown in FIGS. 4, 5, 6 and 7provides a similar mixing action for the working fluid inside the heattransfer element 14.

The heat transfer element 14 has been designed to address all of thedesign criteria discussed above. First, the heat transfer element 14 isflexible and is made of highly conductive material. The flexibility isprovided by a segmental distribution of bellows sections 21,25 whichprovide an articulating mechanism. Bellows have a known convoluteddesign which provides flexibility. Second, the exterior surface area 37has been increased through the use of helical ridges 28,32,36 andhelical grooves 26,30,34. The ridges also allow the heat transferelement 14 to maintain a relatively atraumatic profile, therebyminimizing the possibility of damage to the vessel wall. Third, the heattransfer element 14 has been designed to promote turbulent kineticenergy both internally and externally. The segment design allows thedirection of the grooves to be reversed between segments. Thealternating helical rotations create an alternating flow that results inmixing the blood in a manner analogous to the mixing action created bythe rotor of a washing machine that switches directions back and forth.This mixing action is intended to promote high level turbulent kineticenergy to enhance the heat transfer rate. The alternating helical designalso causes beneficial mixing, or turbulent kinetic energy, of theworking fluid flowing internally.

FIG. 8 is a cut-away perspective view of an alternative embodiment of aheat transfer element 50. An external surface 52 of the heat transferelement 50 is covered with a series of axially staggered,circumferentially overlapping protrusions 54. The staggered, overlappingnature of the protrusions 54 is readily seen with reference to FIG. 9which is a transverse cross-sectional view taken along the line 9—9 inFIG. 8. In order to induce free stream turbulence, the height, d_(p), ofthe staggered protrusions 54 is greater than the thickness of theboundary layer which would develop if a smooth heat transfer element hadbeen introduced into the blood stream. As the blood flows along theexternal surface 52, it collides with one of the staggered protrusions54 and turbulent flow is created. As the blood divides and swirls alongside of the first staggered protrusion 54, it collides with anotherstaggered protrusion 54 within its path preventing the re-lamination ofthe flow and creating yet more turbulence. In this way, the velocityvectors are randomized and free stream turbulence is created. As is thecase with the preferred embodiment, this geometry also induces aturbulent effect on the internal coolant flow.

A working fluid is circulated up through an inner coaxial lumen 56defined by an insulating coaxial tube 58 to a distal tip of the heattransfer element 50. The working fluid then traverses an outer coaxiallumen 60 in order to transfer heat to the exterior surface 52 of theheat transfer element 50. The inside surface of the heat transferelement 50 is similar to the exterior surface 52, in order to induceturbulent flow of the working fluid.

In an alternative embodiment of the invention, one or more of thesegments 20, 22, 24, described with respect to FIGS. 4-7, may bereplaced with a segment having overlapping protrusions 54, such as thosedescribed with respect to FIGS. 8-9.

With reference to FIGS. 10-14, numerous embodiments of a heat transfermechanism for selective heating or cooling of an organ will now bedescribed. The heat transfer mechanism discussed with respect to FIGS.10-14 is similar to the heat transfer element described with referenceto FIGS. 4 and 8 above, but further includes a turbulence-enhancingelement for enhancing the turbulent kinetic energy in the free streamand boundary layer of the heat transfer element 14. It will be shownthat the turbulence-enhancing element may enhance turbulence around theheat transfer element 14 in numerous ways such as, but not by way oflimitation, increasing the velocity of the blood contacting the heattransfer element 14, altering the normal direction of blood flowcontacting the heat transfer element 14, and by increasing the level ofturbulence in the blood flow before the blood reaches the heat transferelement 14, i.e., creating “pre-turbulence.”

With reference to FIGS. 10 and 11, a heat transfer mechanism 70constructed in accordance with an embodiment of the invention will nowbe described. The heat transfer mechanism 70 is located at a distalportion of a supply catheter 12. The heat transfer mechanism 70 includesa heat transfer element 14 located distally of a turbulence-enhancingelement 74. The heat transfer element 14 may be the same as thatdescribed above with respect to FIGS. 4-7 and, thus, will not bedescribed in any further detail. While for purposes of brevity thediscussion herein is directed to a heat transfer element 14, it will bereadily apparent to those skilled in the art that a heat transferelement other than that described with respect to FIGS. 4-7 may be used.For example, the heat transfer element 50 described with respect toFIGS. 8-9 may be used. In the embodiment shown in FIGS. 10 and 11, theturbulence-enhancing element 74 is an expandable micro-balloon 76. Alumen 78 (FIG. 11) is located within the supply catheter 12 forexpanding and contracting the micro-balloon 76 with a fluid, such asair. Although the micro-balloon 76 is described as being expanded andcontracted using air, it will be readily apparent to those skilled inthe art that other fluids, such as saline, may be used. The lumen 78 isin communication with a fluid source at a proximal end of the lumen 78and in communication with an interior 82 of the micro-balloon 76 at adistal end. In an alternative embodiment of the invention, the catheter12 may include more than one lumen for expanding the micro-balloon 76. Aconventional control mechanism may be connected to the proximal end ofthe lumen 78 for controlling expansion and contraction of themicro-balloon 76. Examples of control mechanisms include, but not by wayof limitation, a plunger, a squeezable bladder, or a pump.

The heat transfer mechanism 70 will now be generally described in use.The heat transfer mechanism 70 is positioned in a desired location in apatient's blood vessel 84, upstream from the desired organ to be cooled.During positioning of the heat transfer mechanism 70, the micro-balloon76 is provided in a deflated or collapsed state to facilitate navigationof the catheter 12 and heat transfer mechanism 70 through the patient'svascular system. Once the heat transfer mechanism 70 is in positionwithin the blood vessel, the micro-balloon 76 is expanded by filling itwith fluid. In an expanded state, the micro-balloon 76 restricts theavailable blood flow volume in that region of the blood vessel and thuscauses the blood adjacent the balloon 76 to travel at greater velocitycompared to when the balloon 76 is in a collapsed state. However, asthis blood passes the micro-balloon 76, the blood flow volume area isagain expanded and the blood floods this volume where the heat transferelement 14 is located. Turbulence is enhanced along the heat transferelement, especially at a proximal portion 86 of the heat transferelement 14, by the changing direction of the velocity vectors of theblood flow contacting the segments 20, 22, 24. The blood contacts thesuccessive alternating helical heat transfer segments 20, 22, 24creating alternating flow that results in mixing the blood. This mixingaction promotes high level turbulent kinetic energy to enhance the heattransfer rate between the heat transfer element 14 and the blood. Themicro-balloon 76 further promotes or enhances this high level turbulentkinetic energy by increasing the velocity of the blood contacting theheat transfer element 14.

With reference to FIG. 12, a heat transfer mechanism 90 constructed inaccordance with another embodiment of the invention will now bedescribed. The heat transfer mechanism 90 includes a heat transferelement 14 similar to that described above and a turbulence-enhancingelement 94 in the form of an expandable micro-ring balloon 96. Aninternal lumen similar to the lumen 78 described above with respect toFIG. 11 is located within the catheter 12 and in communication with afluid source and control mechanism at a proximal end for controllinginflation and deflation of the micro-ring balloon 96. A feed lumen 98extends radially from the catheter 12 and communicates a distal end ofthe internal lumen with the micro-ring balloon 96. The micro-ringballoon 96 functions in a similar manner to the micro-balloon 76described above with respect to FIG. 10, except the micro-ring balloon96 cause blood to flow away from the blood vessel wall and into the heattransfer element 14, inducing additional turbulence. Although themicro-ring balloon 96 is shown in communication with a single lumen 98,it will be readily apparent to those skilled in the art that in analternative embodiment, the balloon 96 may be in communication withmultiple lumens. Multiple lumens provide the micro-ring balloon 96 withadditional support and facilitate expansion and contraction of themicro-ring balloon 96 with the vessel 84.

With reference to FIG. 13, a heat transfer mechanism 110 constructed inaccordance with a further embodiment of the invention will be described.The heat transfer mechanism 110 includes a heat transfer element 14similar to that described above and a turbulence-enhancing element 112in the form of axially staggered and circumferentially overlappingprotrusions 115 located on an external surface 116 of the catheter 12.The protrusions 115 may be similar to the protrusions 54 described abovewith respect to FIGS. 8 and 9, except they are preferably locatedproximal of the heat transfer element 14 instead of on the heat transferelement 14. As the blood flows along the external surface 116, itcollides with the staggered protrusions 115 and turbulent flow, i.e.,“pre-turbulence” is initiated. As the blood divides and swirls around astaggered protrusion 115, it collides with another staggered protrusion115 within its path, preventing the re-lamination of the flow andcreating additional turbulence. The turbulent blood then contacts thesuccessive alternating helical heat transfer segments 20-24, creatingalternating flow that results in additional mixing of the blood. Thismixing action promotes further high level turbulent kinetic energy toenhance the heat transfer rate between the heat transfer element 14 andthe blood.

With reference to FIG. 14, a heat transfer mechanism 120 constructed inaccordance with a further embodiment of the invention will be described.The heat transfer mechanism 120 includes a heat transfer element 14similar to that described above and a turbulence-enhancing element 122in the form of a temperature sensor wire 124 such as thermocouple wirehelically wound around an external surface 126 of the catheter 12. Thetemperature sensor wire 124 may include a shrink wrap to hold the wire124 in place. The temperature sensor wire 124 includes a temperaturesensor 128 such as a thermocouple 128 in thermal contact with the heattransfer element 14. In an alternative embodiment, the thermocouple 128may be a thermistor or similar temperature sensor coupled to a wire formeasuring the temperature of the heat transfer element. The thermocouple128 measures the temperature of the heat transfer element 14 forfeedback control of the working fluid temperature. The thermocouple 128may also be disposed distal of the heat transfer element 14 to measurethe temperature of the blood downstream of the heat transfer element 14,e.g., measuring the temperature of the cooled blood. The helically woundthermocouple wire 124 causes blood to swirl over the external surface126 of the catheter, re-directing the blood flow prior to contact withthe heat transfer element 14. The re-direction and swirling of the bloodflow further enhances the amount of turbulence created when the bloodcontacts the successive alternating helical heat transfer segments20-24. In an alternative embodiment of the invention, a thick wire notused for measuring temperature may replace the thermocouple wire 124proximal to the heat transfer element. In a further embodiment, a wire,e.g., thermocouple wire or thick wire, may be helically wrapped around asmooth exterior surface of a heat transfer element such as exteriorsurface 52 described above with respect to FIGS. 8 and 9. The helicallywrapped wire would induce turbulent blood flow around the heat transferelement, enhancing heat transfer in this area.

With reference to FIGS. 15, 16A and 16B, a heat transfer mechanism 140constructed in accordance with an additional embodiment of the inventionwill be described. The heat transfer mechanism 140 includes a heattransfer element 142 and a turbulence-enhancing element 144. The heattransfer element 142 has a primarily smooth external surface 146 adaptedto contact blood within the blood vessel 84. In an alternativeembodiment, the heat transfer element 142 has turbulence-inducingfeatures similar to those described above. The turbulence-enhancingelement 144 includes a turbulence-generating fan 148. The fan 148preferably rotates about an axis 149, coaxial with the axis of thecatheter 12. The fan 148 includes a rotating hub 150 having multipleblades 152 extending therefrom. The fan 148 is adapted to spin uponinfluence of blood flow within the vessel 84. The blades 152 areconstructed to move away from the hub 150 (FIG. 16B) upon influence ofblood flow and towards the hub 150 (FIG. 16A), in a low profileconfiguration, when blood flow lessens or ceases. A low-profileconfiguration means that the blades 152 are located close enough to theexternal surface 146 of the catheter to prevent the fan 148 fromcatching the vasculature upon introduction and removal of the heattransfer element 142. Referring to FIGS. 16A and 16B, bearings 153located between the hub 150 and an external portion 154 of the catheter12 allow the fan 148 to rotate. The hub 150 is appropriately sealed withrespect to the catheter 12 in order to prevent contamination of theblood and protect the bearings 152. The rotating fan 148 induces highlevel turbulent kinetic energy that enhances the heat transfer ratebetween the smooth heat transfer element 142 and the blood.

With reference to FIG. 17, a heat transfer mechanism 160 constructed inaccordance with an additional embodiment of the invention will bedescribed. The heat transfer mechanism includes a heat transfer element142 similar to that described above with respect to FIG. 15 and aturbulence-enhancing element 162. The turbulence-enhancing element 162includes a turbulence generating fan 164 similar to fan 148 describedabove, except the fan 164 includes an internal driving mechanism 166 forrotating the fan 164.

The driving mechanism 166 may include an input lumen 168 and an outputlumen 169 in communication with a pump and fluid source at respectiveproximal ends of the lumens 168, 169 and a fluid drive tunnel 175 atrespective distal ends 176, 177 of the lumens 168, 169. The fan 164includes a rotating hub 170 with multiple blades 172 that move away fromthe hub 170 upon forced rotation of the fan 164 and towards the hub 170when the fan 164 ceases rotation. Bearings 174 are located between thehub 170 and the external portion 154 of the catheter 12. The hub 170 issealed with respect to the catheter 12 in order to prevent contaminationof the blood and protect the bearings 174. The fan 164 includes internalblades 178 located within the fluid drive tunnel 175.

Pressurized fluid such as air is pumped through the input lumen 168 andinto the fluid drive tunnel 175. Air flows through the fluid drivetunnel 175 in the direction of the arrows, causing the fan 164 to rotatevia the internal blades 178. Air exits the fluid drive tunnel 175through the output lumen 169.

The rotating fan 164 induces high level turbulent kinetic energy thatenhances the heat transfer rate between the smooth heat transfer element142 and the blood. It will be readily appreciated that multiplevariations may exist on the heat transfer mechanism 160. For example, inan alternative embodiment, the internal driving mechanism 166 may beconstructed so that the working fluid drives the fan 164 via the fluiddrive tunnel 175 and internal blades 178. Other mechanical drivingmechanisms may be used to drive the fan 164 such as, but not by way oflimitation, a motor coupled to a rotatable drive shaft.

With reference to FIGS. 18A-18C, a heat transfer mechanism 190constructed in accordance with an additional embodiment of the inventionwill be described. The heat transfer mechanism 190 includes a series ofelongated, articulated heat transfer segments 192, 194, 196, which aresimilar to the segments 20, 22, 24 discussed above with respect to FIGS.4-7, connected by flexible joints in the form of tubing sections 198,200. The heat transfer segments 192, 194, 196 serve as a heat transferelement 197 for transferring heat between the blood flow and the heattransfer element 197. The tubing sections 198, 200 are made of aflexible biocompatible material such as a polymer that is seamless andnonporous. The tubing sections 198, 200 are adapted to bend, extend andcompress, which increases the flexibility of the heat transfer element197 so that it is more readily able to navigate through blood vessels.The tubing sections 198, 200 also provide axial compression of the heattransfer element 197, which can limit the trauma when the distal end ofthe heat transfer element 197 abuts a blood vessel wall. The tubingsections 198, 200 are also able to tolerate cryogenic temperatureswithout a loss of performance. During use of the heat transfer mechanism190, working fluid is pulsed through an inner coaxial lumen of an innertube and out of a distal end of the inner tube into an outer lumen (See,for example, FIG. 5, inner coaxial lumen 40, inner tube 42, outer lumen46). As the working fluid is pulsed through the outer lumen, heat istransferred from the working fluid to an exterior surface 202 of theheat transfer element 197. As the working fluid is pulsed through theinner lumen and outer lumen, the flexible tubing sections 198, 200,which include an internal area in fluid communication with the outerlumen, sequentially pulsate or expand, as shown in FIGS. 18B and 18C.The pulsating bellow sections 198, 200 transfer their vibrations to theblood flow, promoting turbulent kinetic energy as the blood flowcontacts the heat transfer element 197. In addition, the expandeddiameter of the flexible bellow sections 198, 200 caused by eachpulsation promotes high level turbulent kinetic energy by increasing thevelocity of the blood contacting the heat transfer element 197 in amanner similar to that described above for the micro-balloon 76.

In an alternative embodiment of the invention, the tubing sections 199,200 may be replaced with micro-balloons, which are connected to separatelumens for individually controlling the pulsation of the balloonsections.

FIG. 19 is a schematic representation of the invention being used tocool the brain of a patient. The selective organ hypothermia apparatusshown in FIG. 19 includes a working fluid supply 10, preferablysupplying a chilled liquid such as water, alcohol or a halogenatedhydrocarbon, a supply catheter 12 and the heat transfer element 14. Theworking fluid supply 10 preferably supplies fluid via a pump (notshown). The supply catheter 12 has a coaxial construction. An innercoaxial lumen within the supply catheter 12 receives coolant from theworking fluid supply 10. The coolant travels the length of the supplycatheter 12 to the heat transfer element 14 which serves as the coolingtip of the catheter. At the distal end of the heat transfer element 14,the coolant exits the insulated interior lumen and traverses the lengthof the heat transfer element 14 in order to decrease the temperature ofthe heat transfer element 14. The coolant then traverses an outer lumenof the supply catheter 12 so that it may be disposed of or recirculated.The supply catheter 12 is a flexible catheter having a diametersufficiently small to allow its distal end to be inserted percutaneouslyinto an accessible artery such as the femoral artery of a patient asshown in FIG. 19. The supply catheter 12 is sufficiently long to allowthe heat transfer element 14 at the distal end of the supply catheter 12to be passed through the vascular system of the patient and placed inthe internal carotid artery or other small artery. The method ofinserting the catheter into the patient and routing the heat transferelement 14 into a selected artery is well known in the art.

Although the working fluid supply 10 is shown as an exemplary coolingdevice, other devices and working fluids may be used. For example, inorder to provide cooling, freon, perflourocarbon or saline may be used.

The heat transfer element of the present invention can absorb or provideover 75 Watts of heat to the blood stream and may absorb or provide asmuch a 100 Watts, 150 Watts, 170 Watts or more. For example, a heattransfer element with a diameter of 4 mm and a length of approximately10 cm using ordinary saline solution chilled so that the surfacetemperature of the heat transfer element is approximately 5° C. andpressurized at 2 atmospheres can absorb about 100 Watts of energy fromthe bloodstream. Smaller geometry heat transfer elements may bedeveloped for use with smaller organs which provide 60 Watts, 50 Watts,25 Watts or less of heat transfer.

The practice of the present invention is illustrated in the followingnon-limiting example.

Exemplary Procedure

1. The patient is initially assessed, resuscitated, and stabilized.

2. The procedure is carried out in an angiography suite or surgicalsuite equipped with fluoroscopy.

3. Because the catheter is placed into the common carotid artery, it isimportant to determine the presence of stenotic atheromatous lesions. Acarotid duplex (doppler/ultrasound) scan can quickly and non-invasivelymake this determinations. The ideal location for placement of thecatheter is in the left carotid so this may be scanned first. If diseaseis present, then the right carotid artery can be assessed. This test canbe used to detect the presence of proximal common carotid lesions byobserving the slope of the systolic upstroke and the shape of thepulsation. Although these lesions are rare, they could inhibit theplacement of the catheter. Examination of the peak blood flow velocitiesin the internal carotid can determine the presence of internal carotidartery lesions. Although the catheter is placed proximally to suchlesions, the catheter may exacerbate the compromised blood flow createdby these lesions. Peak systolic velocities greater that 130 cm/sec andpeak diastolic velocities >100 cm/sec in the internal indicate thepresence of at least 70% stenosis. Stenosis of 70% or more may warrantthe placement of a stent to open up the internal artery diameter.

4. The ultrasound can also be used to determine the vessel diameter andthe blood flow and the catheter with the appropriately sized heattransfer element could be selected.

5. After assessment of the arteries, the patients inguinal region issterilely prepped and infiltrated with lidocaine.

6. The femoral artery is cannulated and a guide wire may be inserted tothe desired carotid artery. Placement of the guide wire is confirmedwith fluoroscopy.

7. An angiographic catheter can be fed over the wire and contrast mediainjected into the artery to further to assess the anatomy of thecarotid.

8. Alternatively, the femoral artery is cannulated and a 10-12.5 french(f) introducer sheath is placed.

9. A guide catheter is placed into the desired common carotid artery. Ifa guiding catheter is placed, it can be used to deliver contrast mediadirectly to further assess carotid anatomy.

10. A 10 f-12 f (3.3-4.0 mm) (approximate) cooling catheter issubsequently filled with saline and all air bubbles are removed.

11. The cooling catheter is placed into the carotid artery via theguiding catheter or over the guidewire. Placement is confirmed withfluoroscopy.

12. Alternatively, the cooling catheter tip is shaped (angled or curvedapproximately 45 degrees), and the cooling catheter shaft has sufficientpushability and torqueability to be placed in the carotid without theaid of a guide wire or guide catheter.

13. The cooling catheter is connected to a pump circuit also filled withsaline and free from air bubbles. The pump circuit has a heat exchangesection that is immersed into a water bath and tubing that is connectedto a peristaltic pump. The water bath is chilled to approximately 0° C.

14. Cooling is initiated by starting the pump mechanism. The salinewithin the cooling catheter is circulated at 5 cc/sec. The salinetravels through the heat exchanger in the chilled water bath and iscooled to approximately 1° C.

15. It subsequently enters the cooling catheter where it is delivered tothe heat transfer element. The saline is warmed to approximately 5-7° C.as it travels along the inner lumen of the catheter shaft to the end ofthe heat transfer element.

16. The saline then flows back through the heat transfer element incontact with the inner metallic surface. The saline is further warmed inthe heat transfer element to 12-15° C., and in the process, heat isabsorbed from the blood cooling the blood to 30° C. to 32° C.

17. The chilled blood then goes on to chill the brain. It is estimatedthat 15-30 minutes will be required to cool the brain to 30 to 32° C.

18. The warmed saline travels back to down the outer lumen of thecatheter shaft and back to the chilled water bath were it is cooled to1° C.

19. The pressure drops along the length of the circuit are estimated tobe 2-3 atmospheres.

20. The cooling can be adjusted by increasing or decreasing the flowrate of the saline. Monitoring of the temperature drop of the salinealong the heat transfer element will allow the flow to be adjusted tomaintain the desired cooling effect.

21. The catheter is left in place to provide cooling for 12 to 24 hours.

22. If desired, warm saline can be circulated to promote warming of thebrain at the end of the therapeutic cooling period.

While the particular invention as herein shown and disclosed in detailis fully capable of obtaining the objects and providing the advantagesstated, it is to be understood that this disclosure is merelyillustrative of the presently preferred embodiments of the invention andthat no limitations are intended other than as described in the appendedclaims.

We claim:
 1. A heat transfer device for transferring heat to or fromsurrounding blood flow to heat or cool the surrounding blood flow,comprising: a catheter capable of insertion into a selected blood vesselin the vascular system of a patient; a heat transfer element attached toa distal portion of said catheter and adapted to transfer heat to orfrom surrounding blood flow to heat or cool the surrounding blood flow,the heat transfer device including more than one heat transfer segmentforming a blood mixing-enhancing element adapted to enhance mixing ofblood flow along said heat transfer element, the blood mixing-enhancingelement including a plurality of exterior surface irregularities, saidsurface irregularities comprise a helical ridge and a helical grooveformed on each said heat transfer segment, and said helical ridge andhelical groove on each said heat transfer segment has an oppositehelical twist to said helical ridge and helical groove on an adjacentheat transfer segment so as to create repetitively changing directionsof blood flow in surrounding blood.
 2. A heat transfer device fortransferring heat to or from surrounding blood flow to heat or cool thesurrounding blood flow, comprising: a catheter capable of insertion intoa selected blood vessel in the vascular system of a patient; a heattransfer element attached to a distal portion of said catheter andadapted to transfer heat to or from surrounding blood flow to heat orcool the surrounding blood flow; and a blood mixing-enhancing elementattached to a distal portion of said catheter and adapted to enhancemixing of blood flow along said heat transfer element, said bloodmixing-enhancing element located proximal of said heat transfer element.3. The device of claim 2, wherein said blood mixing-enhancing elementincludes an expandable micro-balloon.
 4. The device of claim 2, whereinsaid blood mixing-enhancing element includes an expandable micro-ringballoon.
 5. The device of claim 2, wherein said blood mixing-enhancingelement includes multiple axially staggered and circumferentiallyoverlapping protrusions located along an exterior surface.
 6. A heattransfer device for transferring heat to or from surrounding blood flowto heat or cool the surrounding blood flow, comprising: a cathetercapable of insertion into a selected blood vessel in the vascular systemof a patient; a heat transfer element attached to a distal portion ofsaid catheter and adapted to transfer heat to or from surrounding bloodflow to heat or cool the surrounding blood flow; a bloodmixing-enhancing element attached to a distal portion of said catheterand adapted to enhance mixing of blood flow along said heat transferelement, said blood mixing-enhancing element located proximal of saidheat transfer element and including a temperature sensor wire helicallywound around an exterior surface and having a temperature sensor inthermal contact with said heat transfer element for feedback control ofsaid heat transfer element.
 7. The device of claim 6, wherein saidtemperature sensor is a member from the group consisting of thermocoupleand thermistor.
 8. A heat transfer device for transferring heat to orfrom surrounding blood flow to heat or cool the surrounding blood flow,comprising: a catheter capable of insertion into a selected blood vesselin the vascular system of a patient; a heat transfer element attached toa distal portion of said catheter and adapted to transfer heat to orfrom surrounding blood flow to heat or cool the surrounding blood flow;a blood mixing-enhancing element located proximal of said heat transferelement and attached to a distal portion of said catheter and adapted toenhance mixing of blood flow along said heat transfer element; andwherein said catheter includes a longitudinal axis and said bloodmixing-enhancing element includes a fan adapted to rotate about an axiscoaxial with said longitudinal axis of said catheter.
 9. The device ofclaim 8, wherein said fan is adapted to rotate upon fluid flow past thefan.
 10. The device of claim 8, further including a driving mechanismadapted to rotate said fan.
 11. The device of claim 8, wherein said fanincludes multiple blades adapted to have a low profile when said fan isat rest and expand when in motion.
 12. A method for selectivelycontrolling the temperature of blood flow, said method comprising:providing a catheter having a heat transfer element and a bloodmixing-enhancing element attached to a distal portion thereof, the bloodmixing-enhancing element located proximal of the heat transfer element;inserting said catheter through the vascular system of the patient toplace said heat transfer element and blood mixing-enhancing element in adesired blood vessel; creating pre-mixing of blood flow proximal of saidheat transfer element with said blood mixing-enhancing element;circulating fluid into said heat transfer element via an internal lumenof said catheter and via an internal lumen of said heat transferelement; circulating fluid out of said heat transfer element via anexternal lumen of said heat transfer element; and transferring heatbetween said heat transfer element and the blood flow in the bloodvessel, whereby said pre-mixing of blood flow proximal of said heattransfer element enhances the transfer of heat between said heattransfer element and blood.
 13. A method as recited in claim 12, whereinsaid blood mixing-enhancing element is an expandable micro-balloon andis located proximal of said heat transfer element, and creatingpre-mixing by changing the direction of the velocity vectors of theblood flow contacting said heat transfer element with said expandablemicro-balloon.
 14. A method as recited in claim 12, wherein said bloodmixing-enhancing element is an expandable micro-ring balloon and islocated proximal of said heat transfer element, and creating pre-mixingby changing the direction of the velocity vectors of the blood flowcontacting said heat transfer element with said expandable micro-ringballoon.
 15. A method as recited in claim 12, wherein said bloodmixing-enhancing element includes multiple axially staggered andcircumferentially overlapping protrusions located along an exteriorsurface of said catheter proximal to said heat transfer element, andcreating pre-mixing by changing the direction of the velocity vectors ofthe blood flow contacting said heat transfer element with saidprotrusions.
 16. A method as recited in claim 12, wherein said bloodmixing-enhancing element is a temperature sensor wire helically woundaround said catheter proximal to said heat transfer element, saidtemperature sensor wire having a temperature sensor in thermal contactwith said heat transfer element, and said temperature sensor wireenhances mixing by altering the direction of the blood flow contactingsaid heat transfer element and by creating pre-mixing of the blood flowprior to contacting said heat transfer element, and said method furtherincluding feedback controlling the temperature of the circulating fluidbased on the temperature of said heating element.
 17. A method forselectively controlling the temperature of blood flow, said methodcomprising: providing a catheter having a heat transfer element and ablood mixing-enhancing element attached to a distal portion thereof,said heat transfer element including said blood mixing-enhancingelement, said blood mixing-enhancing element including a plurality ofsegments of helical ridges and grooves having alternating directions ofhelical rotation; inserting said catheter through the vascular system ofthe patient to place said heat transfer element and bloodmixing-enhancing element in a desired blood vessel; creating mixing ofblood flow around said heat transfer element by establishingrepetitively alternating directions of helical blood flow with saidalternating helical rotations of said ridges and grooves; circulatingfluid into said heat transfer element via an internal lumen of saidcatheter and via an internal lumen of said heat transfer element;circulating fluid out of said heat transfer element via an externallumen of said heat transfer element; and transferring heat between saidheat transfer element and the blood flow in the blood vessel, wherebysaid mixing of blood flow induced around said heat transfer elementenhances the transfer of heat between said heat transfer element andblood.
 18. A method for selectively controlling the temperature of bloodflow, said method comprising: providing a catheter having a heattransfer element and a blood mixing-enhancing element attached to adistal portion thereof, said blood mixing-enhancing element including atemperature sensor wire helically wound around said catheter, saidtemperature sensor wire having a temperature sensor located distal ofsaid heat transfer element; inserting said catheter through the vascularsystem of the patient to place said heat transfer element and bloodmixing-enhancing element in a desired blood vessel; creating mixing ofblood flow around said heat transfer element with said bloodmixing-enhancing element; circulating fluid into said heat transferelement via an internal lumen of said catheter and via an internal lumenof said heat transfer element; circulating fluid out of said heattransfer element via an external lumen of said heat transfer element;transferring heat between said heat transfer element and the blood flowin the blood vessel, whereby said mixing of blood flow induced aroundsaid heat transfer element enhances the transfer of heat between saidheat transfer element and blood; and feedback controlling thetemperature of the circulating fluid based on the temperature of blooddownstream of said heating element.
 19. A method for selectivelycontrolling the temperature of blood flow, said method comprising:providing a catheter having a heat transfer element and a bloodmixing-enhancing element attached to a distal portion thereof, saidblood mixing-enhancing element including a rotating fan; inserting saidcatheter through the vascular system of the patient to place said heattransfer element and blood mixing-enhancing element in a desired bloodvessel; creating mixing of blood flow around said heat transfer elementwith said rotating fan; circulating fluid into said heat transferelement via an internal lumen of said catheter and via an internal lumenof said heat transfer element; circulating fluid out of said heattransfer element via an external lumen of said heat transfer element;and transferring heat between said heat transfer element and the bloodflow in the blood vessel, whereby said mixing of blood flow inducedaround said heat transfer element enhances the transfer of heat betweensaid heat transfer element and blood.
 20. A heat transfer device fortransferring heat to or from surrounding blood flow to heat or cool thesurrounding blood flow, comprising: a catheter for insertion in a bloodvessel of a patient; a heat transfer element attached to a distal end ofsaid catheter and adapted to transfer heat to or from surrounding bloodflow to heat or cool the surrounding blood flow; a plurality of exteriorsurface irregularities formed on said heat transfer element, saidsurface irregularities being shaped and arranged to create repetitivelychanging directions of blood flow in surrounding blood; and a bloodmixing-enhancing element associated with said heating element forimparting pre-mixing to blood prior to the blood reaching said heatingelement.